Typical sima/delta analog-to-digital converters have a modulator for digitizing a received analog signal and a low pass filter for filtering and decimation. The low pass filter may include an integrator circuit which usually operates at a high system clock frequency and a differentiator circuit which samples down the system clock to a much lower frequency, typically called the baud rate clock. Furthermore, an interpolator circuit can be utilized to provide increased time resolution between two sample points from the differential by a well known method called interpolation.
Typically, two comb filters are used in the differentiator circuit for generating the sampling points that are sent to the interpolator circuit. Typical sampling points are illustrated in the top portion of FIG. 1, where the short vertical arrows indicate sampling points generated from one comb filter, and the long vertical arrows indicate sampling points generated from the other comb filter, whereby the sampling phase of each of the comb filters differs by one system clock cycle as shown. Furthermore, the time between two adjacent sampling points from the same comb filter is the baud period as shown. Coarse sampling phase adjustments are performed by adding or deleting cycles of the system clock to the baud-rate clock. A coarse phase jump is illustrated in the bottom portion of FIG. 1, whereby an advanced coarse jump is made as shown, since the interval of interpolation is one system clock cycle ahead of its previous interval. The other possible jump that could have been made is a retard coarse phase jump, whereby the interval of interpolation is one system clock cycle behind its previous interval.
Fine sampling phase adjustments, that is, phase adjustments which are smaller than the original sampling period (system clock periods) are performed by linearly interpolating between pairs of sampling points (intervals) from the two comb filters. Therefore, the sampling phase is adjusted by simply changing the scale factor in an interpolation equation, as is well known in the art. The main problem with this system, however, is that immediately after a coarse phase jump is made, the comb filters inherently produce a large transient error that will be transmitted to the output of the interpolator circuit. These transient errors exist until the effect of the coarse phase jump has been shifted through the delay elements of the comb filters.
One obvious and simple solution for avoiding transient errors at the output of the interpolation is to utilize a third comb filter. This approach involves adjusting the sampling phase of the third comb filter to the nextanticipated coarse phase jump while the output of the interpolator can then use the third comb filter, along with either the first or second comb filters, to perform interpolation over a new interval, thereby eliminating any transient errors occurring at the output of the interpolator. The three comb filter method is effective, but it is not efficient as a two comb filter approach since comb filters are typically large and expensive to implement.
Thus, a need exists for providing an interpolating decimator with increased resolution while utilizing only two comb filters in the differentiator circuit.